This invention relates to a power assist device for use in apparatus where it is desired to cause a motor to position a heavy object in response to a small manual force being applied to an operating handle. Although the device is useful in many kinds of machines, all illustrative example of its construction and use will be described herein in connection with a diagnostic x-ray table.
A typical use of the new power assist device is in an x-ray table which is equipped with a spot-film device. A spot-film device includes a carriage which is mounted in the body of an x-ray table for being moved longitudinally thereof. Supported on the carriage above the table is an enclosure which may be adapted for accommodating a film cassette and an x-ray image intensifier. The enclosure is usually mounted on the carriage in such manner that it may be shifted to various lateral positions relative to the x-ray table top.
It is customary to have a manually engageable handle attached to the enclosure. The operator may then grasp the handle and push or pull the enclosure to position it laterally as required. Heretofore, a handle has also been provided to enable the operator to apply a force in a longitudinal direction for activating an electric motor that drives the heavy carriage to its desired longitudinal position. The carriage is usually affiliated with a counterweight system which assists in balancing the substantial weight of the carriage and spot-film device when the table is angulated.
In some prior art x-ray tables, the operating handle is mounted for pivoting or sliding through a perceptible distance so that when a force is applied, the handle may actuate potentiometers or switches which select motor speed and direction for longitudinal positioning of the spot film device. In some cases switches have been provided which were merely turned a motor control on and off to select direction. In using this prior art system, the operator is deprived of the naturally expected feeling of proportionality between the magnitude of the manually applied force and the rate and direction in which the spot film device moves. In other words, the expected sensation of having moved a mass is not fed back to an operator who uses one of the prior art handles.
An objective of prior designs was to have the motor drive at a speed that is proportional to the amount of manual force applied to the operating handle. Despite extensive efforts at refinement, however, it has been found that in proportional systems where potentiometers or switches are used to initiate driving action, the operator does not get a true sense or feeling of the massive component moving in proportion to the force which is being applied. Moreover, most prior art systems deactivate brakes coincident with application of a force on the operating handle and the brakes are reset as soon as the force is removed. In systems with or without brakes, however, there is invariably some under-travel or over-travel which, again, results in the operator experiencing an unnatural response. Some operators have objected to prior power driven or power assisted systems as feeling too spongy or too soft and as failing to have the spot film device come to rest where one would expect it to.